1. Field of the Invention
The invention relates to monitoring and controlling the safety of flammables or other hazardous chemicals contained in fuel tanks. One important example is fuel tanks on airplanes. If the ullage (headspace) of a tank is depleted in oxygen, the risk of explosion or reaction is reduced. Inert gas (or an effectively inert gas having a safely low concentration of oxygen) may be used to purge the tank ullage. Measurement of the oxygen concentrations of both the inert gas source and the ullage gas is desirable to assure effective control of an inert gas control system to keep the ullage gas below a safe maximum oxygen concentration. Effective control not only provides for verifying tank safety, but also minimizes fuel penalty and carbon footprint.
2. Description of the Related Art
On-Board Inert Gas Generating Systems (OBIGGS) generally include an oxygen sensor to monitor the Air Separation Module (ASM) output of nitrogen enriched air (NEA) but have not generally monitored the actual oxygen concentration in the ullage of the fuel tanks U.S. Pat. No. 7,625,434 by Tom, Gu, Murphy, and Tang, titled “Enhanced OBIGGS”, discloses measurement of oxygen in the output of an ASM and does not identify the type of oxygen sensor to be used. U.S. Pat. No. 7,574,894 by Austerlitz, Hirshman, Bueter, and Wood, titled “ASM Output Ultrasonic Oxygen Sensor”, discloses measurement of oxygen content of the output of an ASM by using an ultrasonic sensor.
Oxygen concentrations in tanks have been studied with both in situ and sampling sensors revealing various advantages and disadvantages.
In situ sensors located inside the fuel tank must measure oxygen, endure the environment of the tank, and not increase the risk of explosion. In situ electrochemical oxygen sensors have limited life especially in tank vapors and do not measure well at low temperatures. Zirconia oxygen sensors operating at 700 C are the traditional choice but can provide an ignition source. Fluorescence quenching sensors are temperature sensitive and/or pressure sensitive, and the fluorophores may have limited lifetime and saturation effects. U.S. Pat. No. 6,634,598 by Susko, titled “On-board Fuel Inerting System”, discloses a fiberoptic sensor that extends into the tank and which uses chemical fluorescence in combination with a spectrophotometer. Ultrasonic sensors measure the slight difference of the speed of sound in nitrogen and oxygen and are generally less accurate than other sensors types. Each sensor type can have interferences or cross sensitivity with other gases in the tank vapor. In situ sensors having limited lifetime entail difficult service requirements. In the case of in situ optical absorption or fluorescence sensors in tanks, fuel immersion or motion-caused slosh may interfere with the optical sensor window transmission despite metal foam splash guards and droplet removal measures. U.S. Pat. No. 7,352,464 by Chen and Silver, titled “Oxygen Sensor for Aircraft Fuel Inerting Systems”, discloses in-situ detection of oxygen in an aircraft fuel inerting system by using an in-tank oxygen sensor having an optical cavity exposed to the ullage gas and vapor.
Sampling sensors make it possible to remove condensate before measurement of the vapor and to control or correct conveniently the pressure and temperature dependencies. However, sampling systems may require float valves in the fuel tank and the sampling line(s) to prevent ingress of fluid from the tank. They may require flow sensors to detect fuel in the sampling line(s) and initiate purging of the line with inert gas if flow is blocked. They may also require use of check valves, control valves, sampling conditioners, filters, and pumps. If condensate is possible, heated lines, condensate traps, or automated line purging are used. For flammable gas mixtures, the risk of explosion in the sensor is mitigated with measures, for example, as heat and spark avoidance, enclosure inerting, and flame arrestors.
In on-board inert gas generating systems (OBIGGS) for aircraft, typically only the output from the air separation module (ASM) is measured, using zirconia or proposed ultrasonic oxygen sensors, but where ullage gas is not measured. Sampling systems have however been used successfully for OBIGGS flight test validation of the actual ullage concentration, but the measurements taken by these sampling systems have not been used to control ullage purging. Experience with fight test oxygen measurement systems reveal further hazards of sampling systems that require other various risk mitigations. The sealed sensor enclosure is provided with inert gas maintained slightly above ambient pressure. The pump motor may be located outside the sensor enclosure, if flammable vapors are not expected there, for example in the pressurized aircraft areas. The high pressure outlet of a sampling pump inside the enclosure presents a hazard in the case of tubing failure that can be mitigated by a pressure-monitored secondary enclosure. These mitigations result in flight test oxygen measurement systems that are massive.
U.S. Pat. Nos. 7,481,237 and 7,013,905 by Jones, O'Hara, and Greenawalt, titled “System and Method for Monitoring the Performance of an Inert Gas Distribution System”, disclose the use of a zircon or zirconium (more properly referred to as zirconia or zirconium dioxide) oxygen sensor to measure sample flow from a tank's ullage and control an ASM to provide purging gas to a tank's ullage. Since they teach using an oxygen sensor that must operate at an elevated temperature of approximately 700 degrees Centigrade, they also teach conveying the sample to a “remote” measurement location for safety and to remove hydrocarbons from the sample flow before reaching the measurement location of the oxygen sensor. Jones, et al. teaches discharging the sampled ullage gas overboard.
Routine measurement of ullage oxygen concentrations has not been adopted using either in situ or sampling sensors because of these issues. Among the troublesome hazards of sampling systems are also the possible heat and/or sparking of a pump motor and/or of solenoid valves and the possible leaking at a high-pressure port of the pump into hazardous areas. It would be desirable to sample the tank vapor without an electrically powered pump, eliminating potential ignition sources and reducing the size and weight of the oxygen sensor system. It would also be desirable to enable the use of an oxygen sensor that doesn't require operation with a sensing element at a high temperature, and that doesn't have to be remote from the tank being monitored.